Bortezomib is a modified di-peptidyl boronic acid that can inhibit proteosome in organisms. Bortezomib is believed to function as a reversible inhibitor of the chymotrypsin-like activity of the 26S proteasome in mammalian cells. The 26S proteasome is a large protein complex that degrades ubiquitinated proteins. The ubiquitin-proteasome pathway plays a role in regulating the intracellular concentration of specific proteins, maintaining homeostasis within cells. Inhibition of the 26S proteasome prevents this targeted proteolysis, which can affect multiple signaling cascades within the cell. This disruption of normal homeostatic mechanisms can lead to cell death.
Bortezomib is cytotoxic to a variety of cancer cell types in vitro and causes a delay in tumor growth in vivo in nonclinical tumor models, including multiple myeloma. Bortezomib presently is approved for the treatment of multiple myeloma, relapsed multiple myeloma, and mantle cell lymphoma. A variety of combination therapies have been investigated for treating multiple myeloma, in which bortezomib is administered with one or more other biologically active substances, such as lenalidomide, dexamethasone, melphalan, predisone, thalidomide, cyclophosphamide, doxorubicin, vincristine, carmustine, pomalidomide, vorinostat, tanespimycin, and perifosine. Other potential uses of bortezomib also have been reported, including treatment of amyloidosis.
Bortezomib is one of a number of peptidyl boronic acids and peptidyl boronic esters that potentially have biological activity. Non-limiting examples of biological activities reported for peptidyl boronic acids and esters include inhibition of trypsin-like proteases, inhibition of renin, inhibition of the growth of certain cancer cells, and inhibition of proteolytic enzymes. These biological activities may be related to physiological symptoms. For example, proteosome inhibitors can treat infarcts such as occur during stroke or myocardial infarction, and can treat inflammatory and autoimmune diseases. Other possible biological activities of peptidyl boronic acids and esters include reducing the rate of muscle protein degradation, reducing the activity of NF-κB in a cell, reducing the rate of degradation of p53 protein in a cell, inhibiting cyclin degradation in a cell, inhibiting the growth of a cancer cell, inhibiting antigen presentation in a cell, inhibiting NF-κB dependent cell adhesion, and inhibiting HIV replication.
A disadvantage that bortezomib shares with other peptidyl boronic acids and esters is an instability to standard conditions of purification and storage. Boronic acids and esters tend to form anhydrides, including cyclic anhydrides referred to as “boroxines,” during dehydration, which can make it difficult to purify the desired compound. Boronic acids and esters also tend to oxidize in air, which can severely limit their shelf life. Thus, bortezomib typically is difficult to purify, to characterize and/or to formulate into a stable therapeutic product.
One conventional method of increasing the stability of bortezomib involves combining the boronic acid with a sugar or other compound having two or more hydroxyl groups separated by at least two connecting atoms (i.e. C, N, S or O). See, for example, U.S. Pat. No. 6,699,835 to Plamondon et al. It is reported that bortezomib forms a boronate ester with such a di-hydroxyl compound, and that this ester is more stable to air and to dehydration than bortezomib alone. Preferred di-hydroxyl compounds for this stabilization method are disclosed as the reduced sugars sorbitol and mannitol. In a specific embodiment of this method, a mixture of bortezomib, the sugar and a solvent is subjected to lyophilization to remove the solvent, providing a powder containing the bortezomib, the sugar and/or an ester of the bortezomib and the sugar.
This sugar stabilization method has been implemented in the formulation that is commercially available at present and is sold under the VELCADE® trademark. VELCADE® for Injection (Millennium Pharmaceuticals, Inc.; Cambridge, Mass., USA) is currently available as a lyophilized powder containing bortezomib and mannitol. A single dose of VELCADE® includes 3.5 milligrams (mg) bortezomib and 35 mg mannitol. VELCADE® is reconstituted by combining the lyophilized powder with 3.5 milliliters (mL) of 0.9% sodium chloride saline, to provide an injectable solution having a bortezomib concentration of 1 mg/mL.
An alternative method of increasing the stability of bortezomib that has been reported involves combining the compound with a cyclodextrin, a solubilizer, t-butyl alcohol, or one or more of an amino acid, a vitamin, a carboxylic acid and sodium chloride. The “solubilizer” may be a polyoxyethylene-polyoxypropylene copolymer, a fatty alcohol, a fatty alcohol derivative, a fatty acid, or a fatty acid derivative. See PCT Application Publication WO 2010/039762. A lyophilized powder formed from 3.5 mg bortezomib dissolved in 5 mL t-butyl alcohol is reported as having impurity levels below 0.5% (relative to the bortezomib content) after storage for 1 week at 60° C. in a closed container, at 40° C. and 75% relative humidity, or at 25° C. and 60% relative humidity.
The conventional stabilization methods described above have met with mixed success. It would be desirable to have a composition containing bortezomib that is stable for an extended period of time and that can be administered in a conventional way. It also would be desirable for such a composition to retain the therapeutic effectiveness of conventional bortezomib formulations.